Alternator drive fault detection apparatus for multiple alternator systems



M. FLUGSTAD April 1, 195s ALTERNATOR DRIVE FAULT DETECTION APPARATUS FORMULTIPLE ALTERNATOR SYSTEMS 5`Sheets-Sheet l Filed Dec. 24. 1956 ma mm.vm.

mu i L F mi n M A1. MMM,

April 1, 1958 M. FLUGSTAD 2,829,278

ALTERNATOR DRIVE FAULT DETECTION APPARATUS FOR MULTIPLE ALTERNATORSYSTEMS Filed Dec. 24, 1956 y 3 Sheets-Sheet 2 TEP/144702 MPU/PGUCONTROLS y INVENTOR.

Moens FLL/6s TAD BY @wav L-Ldfm A r roem? ya April 1, 1958 M FLUGSTAD2,829,278 ALTERNATR DRIVE FAULT DETECTION APPARATUS FOR MULTIPLEALTERNATOR SYSTEMS Filed Dec. 24. 1956 3 Sheets-Sheet 3 I INVENTOR.(9?{7' l f Mae/s5 F/.uasrAD ALTERNATR DRIVE FAULT DETEC'HN APPA-RA'lllUS FR MULTHPLE ALTERNATR SYSTEMS Morris Flugstad, Seattle, Wash.,assigner' to lioeing Airplane Company, Seattle, Wash., a corporation ofDeia- Ware Application December 24', i956, Serial No. 630,377

7 Claims. (Cl. SW7- 57) This invention relates to alternating currentsupply systems in which a plurality of alternators are paralleled andoperated isochronously. A broad object hereof is the provision ofelectrical apparatus for protecting such g a system againstoverfrequency or underfrequency conditions with attendant unbalance ofreal power delivery ain-ong the alternators. r[he invention is hereinillustratively described by reference to its presently preferred form asapplied to the protection of a system having three similar alternators;however, it will be recognized that the novel apparatus is not limitedto this specific application nor to the details of the illustrativeembodiments.

A more specific object of the invention is the provision of electricaldetection apparatus associated with the respective alternators andoperable to produce an indication or selective control operation, suchas disconnecting a particular alternator and its load from the rest ofthe system, in response to real power overloading or underloading ofthat alternator relative to the other alternators. A principal object isto achieve this result when this is the only abnormal operatingcondition as well as when it is attended by a related deviation ofsystem frequency from, its normal value, such as during faulty operationof a prime mover governor. A related object is the provision of suchdetection apparatus which will distinguish between a systemoverfrequency condition and system underfrequency condition so as topermit initiation of different selective control actions if need be,respectively related to one condition or the other.

A related object is to provide electrical detection apparatus associatedwith the respective alternators and operable to produce an indication orselective control operation in response to an overfrequency orunderfrequency condition inherent in the operation of a particularalternator unit, and to do so independently of the presence or absenceof other system abnormalities such as unbalanced loading of thealternators. In this instance one appropriate selective controloperation is the disconnection of the particular alternator and its loadfrom the remainder of the system. Another such control operation isremoval of drive energy from the prime mover of an offending alternatorunit and actuation of its load circuit breaker if the frequencyabnormality thereof persists after the alternator and its load have beendisconnected from the system. Thus the system will be split up forindependent operation of its units if its load division controlapparatus, normally essential to its frequencyregulated,balanced-loading, isochronous operation, becomes faulty. On the otherhand if the system frequency abnormality results from faulty operationof one particular alternator unit, it alone will be separated from theremainder of the system, and the latter will not be otherwise split.

By providing electrical apparatus for automatically isolating analternator unit causing unbalanced real load division in the system theneed for a mechanical over-- running clutch commonly used betweenalternator and prime mover in aircraft electric power systems iseliminated. This is a distinct improvement because at the high operatingspeeds of aircraft generator prime movers a satisfactory mechanicalclutch of this type is difficult to build and maintain.

Reference is made to the copending application Serial No. 287,482 ofRussell W. Stineman and lohn W. Ward, tiled May i3, 1952, now Patent No.2,636,132, and entitled Alternator Real Power Equalizing Apparatus forMultiple Alternator Systems. In that application there is disclosedapparatus by which real power distribution is substantially equalizedamong paralleled alternators so as to permit their isochronous operationat precisely regulated system frequency. Such apparatus utilizes meansresponsive to differential real currents among the alternators in orderto impose a correction on the individual speed control of any alternatorwhich assumes more or less than its designed share of real load. Thepresent invention is intended to operate in conjunction with and to becompatible with a control of that general type, that is to operate in afrequency-regulated isochronous system having normally balancedalternator loadings. While the present invention also employs adifferential real power loop circuit, it does so for the distinctivepurpose of developing a reference bias signal,

not for purposes of power equalization but for comparison with a systemfrequency error signal, in order to detect the existence and nature ofthe abnormality and to determine which of the alternator units iscausing it.

in its preferred form as herein disclosed the novel apparatus comprises,in association with each of paralleled alternators, frequencydiscrim-inator means producing a frequency abnormality signal related inpolarity and magnitude respectively to the sense and magnitude of anydeparture of system frequency from the normal value, diiferential realpower detecting means producing a signal, added to the first mentionedsignal, and of a polarity and magnitude respectively related to thesense and magnitude of any difference between real power being deliveredby the associated alternator and the average value of real power.delivered by all the alternators, and means responsive to the algebraicsum of the two signals for performing the required indicating orselective control operation. By proper design of these detecting meansthe two signals cancel each other out in the circuits of normalalternator units, but add together to produce a resultant output signalin the case of a faulty unit, thereby providing the required selectivityfor protecting against such faults as a defective governor. However, asindicated above, with such apparatus it will be seen that either afrequency abnormality alone or a real load division imbalance alone canalso produce a net response in said detecting means, so as to split thesystem into its separate units. l In one illustrated embodiment, thecontrol operation consists of the protective operation of the bus tiecircuit breaker of the offending alternator to disconnect the latter andits individual load from the remainder of the system. In a secondillustrated embodiment, additional means are provided operable aftersuch splitting of the system for disconnecting the alternator from itsown load and shutting off its prime mover if a frequency abnormality inthat alternator persists after its isolation from the remainder of thesystem.

These and other features, objects and advantages of the invention willbecome more fully evident from the following description thereof byreference to the accompanying drawings.

Figure l is a simplified schematic diagram of an isochronousload-balanced, frequency-regulated multiple alternator system employingthe invention, the view omitting for simplification certain componentsnormally standard 3 in such systems, including certain protectiveapparatus, voltage regulation apparatus, reactive power equalizationapparatus, etc.

Figure 2 is a simplified schematic diagram of a second embodiment, thisview showing only one alternator unit of a system and omitting forsimplicity certain conventional apparatus normally required for acomplete practical system, but not required in the view for anunderstanding of this invention.

Figures 3 and 4 are magnetic amplifier transfer characteristic diagramsapplicable to Figure 2.

Figure 5 shows a modification of Figure 1.

Referring to Figure 1, the three alternators A1, A2 and A3 are assumedto have equal ratings. The illustration of three alternators in thesystem was selected for purposes of generalization herein but it will berecognized that the invention applies broadly to systems with two ormore alternators. The invention also applies to systems in which thealternators have unequal real load capacity ratings simply bycompensating in any suitable or well known manner for normal differencesin differential real current detected by the novel apparatus.

The alternators are driven by separate prime movers P1, P2 and P3,including frequency regulating governor apparatus subjected to referencefrequency adjustment by control apparatus S1, S2 and S3, respectively,in order to regulate system frequency. The latter apparatus operates tomaintain real power equalization among the alternators by coordinationeected through a loop circuit as in the above-cited co-pending Stinemanand Ward patent application. The connection E will therefore beunderstood to comprise a loop circuit for detecting unbalance of realpower division among the alternators in order to provide correctiveaction through the prime mover governors as in said co-pending patentapplication.

The alternators are illustrated as three-phase machines with groundconnections G1, vG2 and G3, respectively, and output terminals connectedto the respective loads L1, L2 and L3 through three-phase buses B1, B2and B3. Individual alternator circuit breakers AE1, AB2 and AB3 areinterposed in these buses to permit disconnecting the respectivealternators from their loads when necessary. These alternator circuitbreakers may be tripped by controls T1, T2 or T3, respectively. Withlight system loading one or more of these breakers may be tripped sothat full power is carried through the synchronizing bus by thealternators then in operation. Various fault condition detectors such asovervoltage, undervoltage or overcurrent means may be included in thesecontrols. A synchronizing or tie bus SB is provided for interconnectingthe loads of the alternators and thereby synchronizing the system.Individual bus tie circuit breakers BB1, BB?, and BB3, respectively, areinterposed in the connections between the synchronizing bus and theindividual alternators and their loads to permit disconnecting eachalternator and its load from the remainder of the system when required.Suitable or conventional trip controls BTI, BT2 and BT3 may also beprovided for these bus tie breakers. The circuit breakers used in thesystem may be of any suitable or conventional design and may be adaptedfor actuation in any suitable or conventional manner, the specificnature of which, therefore, is not illustrated in the drawing.

ln accordance with the illustrated embodiment of the invention currenttransformers 10, 12 and 14 are respectively linked with one phaseconductor of the alternators A1, A2 and A3 and are connected in seriesin a loop circuit by conductors 16, 18 and 2i?. A differential realpower sensor 22 associated with alternator A1 has input conductors 24and 26 connected to respectively opposite ends of current transformer10. Similarly, the differential real power sensor 28 associated withalternator A2 has input conductors 30 and 32 connected to respectivelyopposite ends of current transformer 12, while differential Cil realpower sensor 34 has input conductors 36 and 38 connected to respectivelyopposite ends of current transformer 14. The voltage of the phaseconductor linked by the current transformer in each instance is alsoapplied as input voltage to the associated differential real powersensor. ln the case of alternator A1 this voltage is applied to sensor22 through input conductors 40 and 42, whereas input conductors 44 and46 apply the corresponding voltage of A2 to sensor 28 and conductors 48and 50 do likewise with respect to A3 and sensor 34. The respectivephase conductors with which the current transformers are inductivelylinked are the same phase conductors in all the alternators.

The differential real power sensors 22, 28 and 34 for the differentalternators are similar, and for that reason only sensor 22 has beenillustrated in detail. In the latter, a voltage divider consisting ofequal resistances 52 and 54 in series is connected across the inputconductors 24 and 26. The primaries of similar transformers 56 and 53are also connected in series between these conductors. The primary oftransformer 59 is connected between input conductors 40 and 42 and thesecondary of this transformer is connected between the respectivejunctions of the resistances 52 and 54 and of the primaries oftransformers 56 and 58. The secondaries of transformers 56 and 58 areconnected across corresponding opposite corners of the similar full-waverectifier bridges 60 and 62, respectively. The intermediate oppositecorners of bridges 60 and 62 are series-connected in buckingrelationship across the voltage divider consisting of equal resistances64 and 66 shunted by equal filter condensers 68 and 70, respectively.The net voltage appearing across the serially connected resistances 64and 66 is applied to the output terminals 72 and 74 of differential realpower sensor 22. This voltage is designated EP. In the illustrated formof the circuit the polarity of this voltage is as indicated in thefigure if the real power being delivered by alternator A1 exceeds theaverage real power being delivered by all the alternators. The oppositepolarity would occur if the reverse real power imbalance occurred withrespect to this alternator. The voltage EP is normally zero, that is, iszero when the system real power is divided equally among thealternators, but becomes approximately proportional to any real powerdelivery excess or deficiency of alternator A1.

That differential real power sensor 22 functions to the end describedabove may be observed from the fact that the current transformers 10, 12and 14 are connected in a series loop so that any component of currentflowing through voltage divider 52, 54 in phase or 180 degrees out ofphase with voltage across conductors 40, 42 will be proportionalrespectively to the amount by which real power delivery of alternator A1exceeds or is less than the average real power delivery of all thealternators. If the alternator loads are balanced the only voltageapplied to the primaries of transformers 56 and 58, respectively, willbe that applied by the secondary of voltage transformer 59. Under thiscondition no voltage appears between output terminals 72 and 74 due tothe bucking relationship of the rectifiers 60 and 62. However, if A1assumes more or less than its proper share of real load, a component ofvoltage will appear across voltage divider 52, 54, half of which willadd vectorially to the secondary voltage of transformer 59 and the otherhalf of which will subtract vectorially from this secondary voltage.Consequently a direct voltage of one polarity or the other will appearacross output terminals 72, 74 depending on whether alternator A1 isrelatively overloaded or underloaded.

The above explanation assumes that the alternators operate intosubstantially balanced three-phase loads a1- though if they are notbalanced the control apparatus represented by the present invention willnot be totally inoperative but will be less sensitive or less effectivefor its intended purpose. Of course, in the case of a single phasesystem this is not a factor.

The alternators A1, A2 and A3 also have associated with them respectivefrequency discriminators 76, 78 and 80 energized by voltage from one ofthe phases of each alternator. In the example, discriminator 76 isenergized by the voltage between conductors and 42, discriminator 78 bythe voltage between conductors 44 and 46, and discriminator 80 by thevoltage between conductors 48 and 50. The frequency discriminators areof similar form and therefore only the details of discriminator 76 areillustrated. Frequency discriminator 76 preferably is of conventionalform. It comprises two similar transformers 82 and 84 whose primariesare connected in series across conductors 40 and 42. The secondary oftransformer 82 is tuned by shunt condenser 86 to a frequency displacedin one sense from the normal system operating frequency, while thesecondary of transformer 84 is tuned by shunt condenser S8 to afrequency displaced in the opposite sense, but by an equal amount fromsaid operating frequency. A full wave bridge rectifier 90 is connectedby opposite corners across condenser 36 and a similar rectiiier 92across condenser 88. The intermediate corners of rectifier bridge 90 areshunted by a parallel filter condenser 94, and resistance 96, and theintermediate corners of bridge 92 by a similar condenser 98 andresistance 100. The resistances are series-connected and their outerends are connected respectively to discriminator output terminals 102and 104.

rihe discriminator operates in the usual manner to produce an outputvoltage EF which is proportional to frequency departure from the normalvalue and of a polarity related to the sense of departure. Such Voltageis added algebraicaly to the voltage EP of the associated differentialreal power sensor 22 for application to the input of the associatedamplifier 106. Similar amplifiers 108 and 110 receive the outputs fromdiscriminators 78 and 80, respectively. As will be described, a certainpolarity relationship between the output voltage of the frequencydiscriminator and associated real power sensor must be observed. In theexample the polarity of EF should be as illustrated when systemfrequency exceeds the normal value. Any output signals from amplifiers106, 108 and 110 are applied through the respective connections 112,11,4 and 116 to the bus tie circuit breakers BB1, BBZ, and BB3 foractuation thereof. Under normal operating conditions both voltages Epand EF will be zero, so that the associated bus tie circuit breaker willremain unactuated. However, if either the voltage EP or the voltage EFbecomes appreciable while the other remains at zero, or if the twovoltages become appreciable and add together to produce a resultantother than zero, which they do only in the case of an alternator unitwhich causes unbalanced load division and attendant frequency deviation,the related bus tie circuit breaker will be actuated to split the systemas to the offending alternator. In the case of normally operatingalternators the sum of EF and Ep is zero.

In order to assure mutual cancellation of EF and EP in the detectorsassociated with nonfaulty alternator units during malfunctioning of theprime mover governor of another unit or during some other malfunctioningof the latter causing abnormal system frequency and unbalanced loadingamong the alternators certain design relationships must be observed.These may be expressed as follows for the ideal cause.

APrP-Pau (EPA-EF) =K(iAf) -i-KSAP where (EP-l-EF)=direct voltage intoamplier 06 (positive values indicate overfrequency and overloading ofA1) Pl=real power delivery by A1 Gil Pav=average real power delivery byparalleled alternators A1, A2 and A3 Af=frequency deviation from normalvalue Kzvolts/lAfii. e. sensitivity of frequency discriminator 76,wherelAlis magnitude of frequency deviation of .zitif/Enpii e.sensitivity of real load division control means S (Figure l), that isthe amount of change in prime mover governor reference frequency settingwhich it maires in response to a given MP1, where |AP is magnitude ofpower division imbalance AP.

Thus it will be seen that operation of the detecting means of thepresent system should be compatible with that of the load divisioncontrol means S of the system and that the differential real powersensing means of any alternator should have a gain factor related tothat of the associated frequency discriminator.

Whenever any of the bus tie breakers BEF., BBZ or BB3 are tripped todisconnect the associated alternator and its load from the remainder ofthe system the corresponding contactor CB1, CB2 or CBS is also ciosedand shorts across the associated current transformer- 10, 12 or 14. Thusthe latter is removed from the current transformer loop and does notinterfere with its operation as a part of the remaining system. Similarcontactors CAll, CA2 and GA3 in the alternator circuit breakers ABl, ABZand ABE perform a similar function when the latter circuit breakers aretripped.

in the system of Figure l the amplifiers 106, 108 and it@ are of anysuitable or conventional type responsive to input signals of eitherpolarity and predetermined magnitude for tripping the associated bus tiecircuit breakers through the respective connectors 112, lit-i and 1M.

ln the modification of Figure 2, wherein certain reference numeralsdesignate components and devices similar to those designated by likereference numerals in Figure l, the conditions of overfrequency andattendant excessive real power loading of an alternator and ofunderfrequency and attendant deficient real power loading of thealternator are detected separately. Alternators A2 and A3 are not shownin the figure but it will be obvious from the drawing that are connectedin the system just as in Figure l and that each wiil have sensing andprotective apparatus similar to that illustrated in association withaiternater Ai and connected therewith in the manner and to the endsillustrated and described.

InFigure 2 the control winding 122 of magnetic ampliiier lill?, isconnected through rectifier 124 to the respective output terminals 74and 104 so as to be energized by a resultant voltage representing theaigebraic sum of EP and EF. The polarity of rectifier 124 is such thatcontroi winding 12.2 is so energized only in response to the polarity ofthe resultant of EP plus EF which corresponds to the overfrequency,overload condition of alternator Al.. Similarly the control winding i126of magnetic amplfiicr 12d is energized through rectifier 12S from thesame output terminals, with the relative polarity of rectiiier 12S beingopposite that of rectifier 124. Thus control winding 126 is energizedoniy when the sum EP plus EF has a poiarity corresponding to theunderfrequency, underload condition of alternator A1.

Magnetic amplifiers M8 and 1Z0 are of conventional form. Ampliiier litiincludes a reactance or primary winding 130 connected in series withoppositely poled rectitiers 132 and 334 across energizing conductors 40and 42. A similar primary winding 136 is likewise connected in serieswith oppositely poled rectiiers 13S and 140 across the same conductors.The output terminals OF of this rectifier are connected to correspondingends of the two primary windings as shown. A bias winding 143 isenergized from conductors 40 and 42 through the full-wave rectifierbridge M2 feeding into a network consisting of series inductance 144,shunt resistance 146, series resistance 143 and shunt voltage regulatortube which network both filters and stabilizes the rectified.l

voltage applied to the bias winding. A variable resistance 152 connectedin series with the bias winding may be adjusted to establish the biascurrent flowing through the winding so as to place the transfercharacteristic of the magnetic amplifier at the desired control point asshown in Figure 4. Thus, the bias on ampler 113 is set so that theoutput signal at terminals OF jumps suddenly from a relatively low valueto a relatively high value when the sum EP plus EF, during increasethereof in the positive sense shown, reaches a predetermined positivethreshold value. A delay winding 127 shunted by variable resistance 129prevents any normal system frequency transient currents in winding 12bfrom producing a signal at terminals OF.

Ampliiier 12) is similar to amplifier 118, including primary Winding 154and rectifiers 155 and 152i, primary winding 16) and rectiiiers 162 and16:1, bias winding 166, and variable resistance 168 connected in serieswitli this bias winding across the VR tube b. The resulting transfercharacteristic of magnetic ampliiier 120, as shown in Figure 3, may beset at the desired control point by t'ne adjustment of resistance 16S sothat the output signal at terminals UF jumps suddenly from a relativelylow value to a relatively high value at a predetermined negativethreshold value of the algebraic sum EP plus EF. Amplifier 12u alsoincludes delay Winding 131 and variable shunting resistance 133.

Lead conductors 170 and 172 extend from output terminals OF to the bustie circuit breaker BB1 for tripping the circuit breaker and therebydisconnecting the alternator and its load L1 from the remainder of thesystem in response to the increase of output signal at terminals OF to amaterial value. Conductors 174 and 176 similarly connect outputterminals UF to the bus tie circuit breaker BB1 for tripping the same inresponse to the increase of output signal at terminals UF to a materialvalue. Such increase of output signal at terminals OF is indicated byillumination of lamp 173 whereas that which occurs at terminals UF isindicated by illumination of lamp 181i.

A time delay relay 182 is connected in series with a direct voltagesource 184i across normally open bus tie breaker contactor DB1. Closureof such contactor automatically when the bus tie circuit breaker istripped actuates relay 182 after the time delay inherent in the relay.

The delay period is selected to allow sufficient time for the alternatorunit A1 to restabilize itself in frequency after it is isolated byoperation of BB1. if it still operates at abnormal frequency thereafter,the persisting signal at OF will actuate a prime mover shutoif device yM1 through the now closed contacts 182e and 182b of relay 182. As aresult, the prime mover shutoff device removes drive power from theprime mover P1. This shutoff device may be of any suitable typedepending upon the type of prime mover and the manner of driving l thesame. Such shutoff device may either remove all drive power from theprime mover or may reduce the drive power to a predetermined minimumvalue, depending upon the nature of the apparatus.

if an output signal occurs at terminals UF the resultant tripping of bustie circuit breaker BB1 causes closure of normally open contactor BB1therein to form an energizing circuit for time delay relay 19t). Thisenergizing circuit includes the voltage source 192, the contactor EB1and the connecting leads 194 and 196. The time delay period inherent intime delay relay 190 is such that if the tripping of the bus tie circuitbreaker is effective to remove the cause of the underfrequencycondition, represented by the signal at terminals UF, a suiiicientperiod of time will be allowed in which the alternator A1 will becomestabilized at the normal operating frequency. Consequently, actuation ofthis relay at the end of such period by continued closure of contactorBB1, connecting the actuating coil of alternator circuit breaker ABl toconductors'lilf-. and 176 through the relay contacts e and 190i, doesnot'result in tripping of the alternator circuit breaker. However, ifthe underfrequency condition persists beyond the period of actuation oftime delay relay 19t), then the alternator circuit breaker will betripped by the underfrequency signal at terminals UF. Moreover, closureof relay contacts 190a and 190i) under the latter condition will form aconnection, through conductors 193 and 260 to the prime mover shutoffM1, which connection will actuate the latter.

Thus it -viii be seen that if actuation or tripping of the bus tiecircuit breaker BB1 does not relieve the overfrequency or theunderfrequency condition causing that result, the prime mover shutoff M1will be actuated, allowing a predetermined intervening time delay period(which may or may not be the same for the two conditions) forrestabilization of the isolated alternator, should it be capable ofrestabilization, at the correct operating frequency.

It will become evident from a study of Figure 2 that the protectiveapparatus responds in substantially the same way to an underfrequencycondition unaccompanied by a subnormal share of real power delivery byalternator A1, as it does to an underfrequency condition accompanied bythe attendant subnormal share of real power delivery by that alternatorwhich takes place, for example, when the governor of its prime movercauses the prime mover to operate at subnormal speed. Likewise it willbe evident that the apparatus responds in substantially the same way toan overfrequency condition unaccompanied by an abnormal or excessiveshare of real power delivery by alternator A1, as it does to anoverfrequency condition accompanied by an excessive share of real powerdelivery by alternator A1 caused, for example, by excessive drive torqueapplied by the prime mover P1 due to a fault in its governor. ContactorCB1 shorts out current transformer 10 in response to actuation of bustie circuit breaker BB1, as does contactor CAI in response to actuationof alternator circuit breaker ABI, as in Figure l. With the currenttransformer 10 shorted out by either of these contactors, the remainderof the system may continue to operate as a system. Moreover, if the bustie circuit breaker BB1 has been tripped the resulting short circuitacross current transformer 10 during isolated operation of alternator A1supplying its own load L1 continues to be under the protection of theoverfrequency and underfrequency sensing apparatus. This is trueinasmuch as, while EP becomes zero, yet EF assumes an appreciable valueof one polarity or the other as frequency of the alternator departsabove or below the assigned vaine. Thus, either an underfrequency oroverfrequency condition during isolated operation will actuate the primemover shutoff M1. Should an open circuit occur in the power divisionregulation loop E during parallel operation, a resulting excessivesystem frequency droop (droop will depend on total system real load)will split the system into its units for independent isolated operationthereof.

In Figure 5 the arrangement of Figure 1 is employed with a singlefrequency discriminator 76', similar to '76, serving in the detectionmeans for all of the alternators. lts energization is derived from thesynchronizing bus SB. Such an arrangement offers simplicity but is notconsidered as reliable nor as versatile as the arrangements in whichseparate frequency discriminators are employed.

I claim as my invention:

l. in an alternating current system including a plurality of alternatorunits interconnected electrically in parallel and having prime moverswith means controlling the same normally for predetermined load divisionamong the alternators and for isochronous, frequency-regulated operationof said alternators, the combination for detecting malfunctioning of anyalternator unit affecting system frequency and load division among thealternators, comprising frequency detecting means connected to saidalternators and adapted to produce an output signal substantiallyproportional in magnitude to a deviation of frequency thereof from apredetermined value, said output signal being of one polarity for anupward frequency deviation and of the opposite polarity for a dowardfrequency deviation, differential real power sensing means for therespective alternators, including real current sensing means connectedto the associated alternator and to the similar sensing means of theother alternators and adapted thereby to produce an output signalsubstantially proportional in magnitude to a difference between realpower delivery by its associated alternator and average real powerdelivery by all of said interconnected alternators, said latter outputsignal being of one polarity for one sense of difference and of theopposite polarity for the opposite sense of difference, meansinterconnecting the respective outputs of said differential real powersensing means with the output of said frequency detecting means wihpredetermined polarity relationship therebetween causing an outputsignal from the differential real power sensing means of anymalfunctioning alternator unit to add the output signal from saidfrequency detecting means, while the simultaneously occurring outputsignals from the differential real power sensing means of the remainingalternator units subtract from the frequency detecting means outputsignal, and separate detector means responsively connected to each ofsaid differential real power sensing means and selectively responsive tothe summation of its output signal added to said frequency detectingmeans output signal.

2. In an alternating current system including a plurality of alternatorunits interconnected electrically in parallel and having prime moverswith means controlling the same normally for predetermined load divisionamong the alternators and for isochronous, frequency-regulated operationof said alternators, the combination for protesting the system againstmalfunctioning of any alternator unit affecting system frequency andload division among the alternators, comprising frequency detectingmeans connected to said alternators and adapted to produce an outputsignal substantially proportional in magnitude to a deviation offrequency thereof from a predetermined value, said output signal beingof one polarity for an upward frequency deviation and of the oppositepolarity for a downward frequency deviation, differential real powersensing means for the respective alternators, including real currentsensing means connected to the associated alternator and to the similarsensing means of the other alternators and adapted thereby to produce anoutput signal substantially proportional in magnitude to a differencebetween real power delivery by its associated alternator and averagereal power delivery by all of said interconnected alternators, saidlatter output signal being of one polarity for one sense of differenceand of the opposite polarity for the opposite sense of difference, meansinterconnecting the respective outputs of said differential real powersensing means with the output of said frequency detecting means withpredetermined polarity relationship therebetween causing an outputsignal from the differential real power sensing means of anymalfunctioning alternator unit to add to the output signal from saidfrequency detecting means, while the simultaneously occurring outputsignals from the differential real power sensing means of the remainingalternator units subtract from the frequency detecting means outputsignal, separate circuit breaker means operable to disconnect each ofsaid alternators from the remaining alternators and to disconnect thereal current sensing means of said alternator effectively from therelated frequency detecting means and from the real current sensingmeans of said remaining alternators, and separate detector meansresponsively connected to each of said differential real power sensingmeans, and operatively connected to the associated circuit breakermeans, said de- 10 tector means being selectively responsive to thesummation of the outputsignal of the associated differential real powersensing means added to that of said frequency detecting means, tooperate the associated circuit breaker means.

3. The combination defined in claim 2, wherein the frequency detectingmeans comprise a plurality of similar but separate frequency detectingmeans connected to the respective alternators and having individualoutputs interconnected with the, outputs of the respectively associateddifferential real power sensing means.

4. The combination defined in claim 3, a separate load for eachalternator, thev circuit breaker means of each such alternator beingadapted to disconnect the alternator and its load from the remainingalternators, the detector means of each alternator being responsive alsoto an output signal from the frequency detector means alone, andseparate means operable to remove power from the respective primemovers, said latter means being connected to the detector means of therespectively associated alternators and adapted for operation therebyfollowing operation of the associated circuit breaker means.

5. In an alternating current system including a plurality of alternatorunitsinterconnected electrically in parallel and having prime moverswith means controlling the same normally for predetermined load divisionamong the alternators and for isochronous, frequency-regulated operationof said alternators, the combination for detecting malfunctioning of anyalternator unit affecting system frequency and load division among thealternators, comprising frequency detecting means connected to saidalternators and adapted to produce an output signal substantiallyproportional in magnitude to a deviation of frequency thereof from apredetermined value, said output signal being of one polarity for anupward frequency deviation and of the opposite polarity for a downwardfrequency deviation, differential real power sensing means for therespective alternators, including real current sensing means connectedto the associated alternator and to the similar sensing means of theother alternators and adapted thereby to produce an output signalsubstantially proportional 1n magnitude to a difference between realpower delivery by its associated alternator and average real powerdelivery by all of said interconnected alternators, said latter outputsignal being of one polarity for one sense of difference and of theopposite polarity for the opposite sense of difference, meansinterconnecting the respective outputs of said differential real powersensing means with the output of said frequency detecting means withpredetermined polarity relationship therebetween causing an outputsignal from the differential real power sensing means of anymalfunctioning alternator unit to add to the output signal from saidfrequency detecting means to produce a resultant signal of one polarityfor over-frequency and relative overloading of the alternator and of theopposite polarity for underfrequency and relative underloading of thealternator, While the simultaneously occurring output signals from thedifferential real power sensing means of the remaining alternator unitssubtract from the frequency detecting means output signal, and separatedetector means responsively connected to each of said differential realpower sensing means and selectively responsive to the resultant of itsoutput signal added to said frequency detecting means output signal,said detector means including two separate portions each sensitive toresultant signals of a different polarity.

6. The combination defined in claim 5, a separate load connected to eachof the alternators, a separate alternator circuit breaker interposedbetween each alternator and its load and operable to disconnect thealternator from its load, tie bus means interconnecting the alternatorloads, a separate bus tie circuit breaker interposed between eachalternator load and said tie bus means and operable to disconnect eachalternator and its load from.

ll the other alternators, means operatively connecting both portions ofthe respective detector means to the bus tie circuit breaker means ofthe respectively associated alternators to Operate the bus tie circuitbreaker of any such malfunctioning alternator unit, time delay meansassociated with each alternator, controlled by operation of theassociated bus tie circuit breaker means and operatively connecting theassociated detector means portion which is sensitive to theunderfrequency condition to the associated alternator circuit breaker,thereby to effect operation of the alternator circuit breaker of anyalternator unit which operates in the underfrequency condition forlonger than a predetermined time delay period following oper- Noreferences cited.

U. Su DEPARTMENT OF COMMERCE PATENT OFFICE CERTIFICATE F Fammi Na,238293278 Morris Flugstad Signed and sealed this 26th day of" 19580(SEAL) Atestzs KARL Ho AXLNE OEM C. WATSW wasting Officer' immane? afPatente U. S DEPARTMENT OF COMMERCE PATENT OFFICE CERTIFICATE OFCORRECTION Patent No., 829,278

April l, 1958 Morris Flugstad Signed and sealed this 20th day of May19580 (SEAL) A'best:

KARL H; AXLINE ROBERT c. WATSON Attuting Officer' isaioner of Patents

